Concrete is a composite material usually comprising a mineral-based hydraulic binder which acts to adhere mineral particulates together in a solid mass; those particulates may consist of coarse aggregate (rock or gravel), fine aggregate (natural sand or crushed fines), and/or unhydrated or unreacted cementitious or pozzolanic material. Concrete typically is made from portland cement (“PC”), water and aggregate. Curing concrete requires two elements: suitable temperature and water. To achieve maximum strength, all cement particles must be hydrated. The initial process of hydration is exothermic; it generates a considerable amount of energy called the “heat of hydration.” Fluid (plastic) concrete is poured in various forms or molds. These prior art uninsulated forms are exposed to the environment, and, therefore, the energy from the heat of hydration is generally lost to the environment in the first 8-36 hrs. In the next few days, most of the free moisture is also lost from the concrete. Therefore, the two elements required to hydrate the cement are often lost during the initial stage of concrete curing. Thus, the cement may never fully hydrate, and, therefore, may never achieve its maximum strength. Industry practice indicates that portland cement concrete achieves 90% of its maximum strength under ideal curing conditions in about 28 days.
Portland cement manufacture causes environmental impacts at all stages of the process. During manufacture, a metric ton of CO2 is released for every metric ton of portland cement made. Worldwide CO2 emissions from portland cement manufacture amount to about 5%-7% of total CO2 emissions. The average energy input required to make one ton of portland cement is about 4.7 million Btu—the equivalent of about 418 pounds of coal. The production of portland cement is therefore highly energy intensive, accounting for about 2% of primary energy consumption globally. In 2010 the world production of hydraulic cement was about 3,300 million tons.
Concrete can also be made with slag cement (“SC”) and various other pozzolans, such as fly ash (“FA”), but are not frequently used. Slag cement and fly ash generate relatively low amounts of heat of hydration, which result in extremely slow setting times and strength gain. Slag cement and fly ash can be mixed with portland cement but industry practice in building construction limits use of slag cement and fly ash to no more than 30% replacement of portland cement and only during warm weather conditions. Concrete made with slag cement and fly ash may take up to 90 days to achieve 80%-90% of maximum strength. Mass concrete structures use more slag cement and fly ash, replacing up to 80% of portland cement, as a means to reduce the heat of hydration to reduce cracking. Slag cement and fly ash use less water to hydrate, may have finer particles than portland cement and produce concretes that achieve higher compressive and flexural strength. Such concrete is also less permeable, and, therefore, structures built with slag cement and fly ash have far longer service lives or lifecycles.
Slag cement is obtained by quenching molten iron slag (a by-product of iron and steel-making) from a blast furnace in water or steam, to produce a glassy, granular product that is then dried and ground into a fine powder. Slag cement manufacture uses only 15% of the energy needed to make portland cement. Since slag cement is made from waste materials; no virgin materials are required and the amount of landfill space otherwise used for disposal is reduced. For each metric ton of pig iron produced, approximately ⅓ metric ton of slag is produced. In 2009, worldwide pig iron production was about 1.211 billion tons. There was an estimated 400 million tons of slag produced that could potentially be made into slag cement. However, only a relatively small percentage of slag is used to make slag cement in the USA.
Fly ash is a by-product of the combustion of pulverized coal in electric power generation plants. When pulverized coal is ignited in a combustion chamber, the carbon and volatile materials are burned off. However, some of the mineral impurities of clay, shale, feldspars, etc. are fused in suspension and carried out of the combustion chamber in the exhaust gases. As the exhaust gases cool, the fused materials solidify into spherical glassy particles called fly ash. The quantity of fly ash produced worldwide is growing along with the steady global increase in coal use. According to Obada Kayali, a civil engineer at the University of New South Wales Australian Defense Force Academy, only 9% of the 600 million tons of fly ash produced worldwide in 2000 was recycled and even smaller amount used in concrete; most of the rest is disposed of in landfills. Since fly ash is a waste product, no additional energy is required to make it.
Concrete can also be made from a combination of portland cement and pozzolanic material or from pozzolanic material alone. There are a number of pozzolans that historically have been used in concrete. A pozzolan is a siliceous or siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties (ASTM C618). The broad definition of a pozzolan imparts no bearing on the origin of the material, only on its capability of reacting with calcium hydroxide and water. The general definition of a pozzolan embraces a large number of materials, which vary widely in terms of origin, composition and properties. Both natural and artificial (man-made) materials show pozzolanic activity and are used as supplementary cementitious materials. Artificial pozzolans can be produced deliberately, for instance by thermal activation of kaolin-clays to obtain metakaolin, or can be obtained as waste or by-products from high-temperature process, such as fly ashes from coal-fired electricity production. The most commonly used pozzolans today are industrial by-products, such as slag cement (ground granulated blast furnace slag), fly ash, silica fume from silicon smelting, highly reactive metakaolin, and burned organic matter residues rich in silica, such as rice husk ash. Alternatives to the established pozzolanic by-products are to be found on the one hand in an expansion of the range of industrial by-products or societal waste considered and on the other hand in an increased usage of naturally occurring pozzolans. Silica fume (also known as microsilica) is an amorphous form of silicon dioxide. Silica fume consists of sub-micron spherical primary particles.
Natural pozzolans are abundant in certain locations and are used as an addition to portland cement in some countries. The great majority of natural pozzolans in use today are of volcanic origin. Volcanic ashes and pumices largely composed of volcanic glass are commonly used, as are deposits in which the volcanic glass has been altered to zeolites by interaction with alkaline waters. Deposits of sedimentary origin are less common. Diatomaceous earths, formed by the accumulation of siliceous diatom microskeletons, are a prominent source material here. Romans used volcanic ash mixed with lime to make concrete over 2,000 years ago.
Concrete walls, and other concrete structures and objects, traditionally are made by building a form or a mold. The forms and molds are usually made from wood, plywood, metal and other structural members. Unhardened (plastic) concrete is poured into the space defined by opposed spaced form members. Once the concrete hardens sufficiently, although not completely, the forms are removed leaving a concrete wall or other concrete structure, structural member or concrete object exposed to ambient temperatures. Concrete forms are typically made of various types of plywood or metal supported and/or reinforced by a frame structure. These forms are not insulated which means that concrete contained in such forms is exposed to the elements during the curing process. During the curing process, the heat generated by the hydration of cement is lost to the environment. This often makes the curing of the concrete a slow process and the ultimate strength difficult to control or predict. To compensate for these losses and increase the rate of setting and strength development, larger amounts of portland cement are used than otherwise would be necessary.
The curing of plastic concrete requires two elements, water and heat, to fully hydrate the cementitious material. Cement hydration is an exothermic process. This heat is produced by the hydration of the portland cement, or other pozzolanic or cementitious materials, that make up the concrete paste. Initially, the hydration process produces a relatively large amount of heat. Concrete placed in conventional forms (i.e., uninsulated forms) loses this heat of hydration to the environment in a very short time, generally in the first 8-36 hours, depending on the ambient temperature. Also, concrete placed in conventional forms may not reach its maximum potential temperature. As the hydration process proceeds, relatively less heat of hydration is generated due to slowing reaction rates. At the same time, moisture in the concrete is lost to the environment. If one monitors the temperature of concrete during the curing process, it produces a relatively large increase in temperature, which then decreases relatively rapidly over time. This chemical reaction is temperature dependent. That is, the hydration process, and consequently the strength gain, proceeds faster at higher temperature and slower at lower temperature. In conventional forms, both heat and moisture are lost in a relatively short time, which makes it difficult, or impossible, for the cementitious material to fully hydrate, and, therefore, the concrete may not achieve its maximum potential strength.
Conventional forms or molds provide little or no insulation to the concrete contained therein. Therefore, heat produced within the concrete form or mold due to the hydration process usually is lost through a conventional concrete form or mold relatively quickly. Thus, the temperature of the plastic concrete may initially rise 20 to 40° C., or more, above ambient temperature due to the initial hydration process and then fall relatively quickly to ambient temperature, such as within 8 to 36 hours depending on the climate and season and size of the concrete element. This initial relatively large temperature drop may result in significant concrete shrinkage and/or thermal effects which can lead to concrete cracking. The remainder of the curing process is then conducted at approximately ambient temperatures, because the relatively small amount of additional heat produced by the remaining hydration process is relatively quickly lost through the uninsulated concrete form or mold. The concrete is therefore subjected to the hourly or daily fluctuations of ambient temperature from hour-to-hour, from day-to-night and from day-to-day. Failure to cure the concrete under ideal temperature and moisture conditions affects the ultimate strength and durability of the concrete. In colder weather, concrete work may even come to a halt since concrete will freeze, or not gain much strength at all, at relatively low temperatures. By definition (ACI 306), cold weather conditions exist when “ . . . for more than 3 consecutive days, the average daily temperature is less than 40 degrees Fahrenheit and the air temperature is not greater than 50 degrees Fahrenheit for more than one-half of any 24 hour period.” Therefore, in order for hydration to take place, the temperature of concrete must be above 40° F.; below 40° F., the hydration process slows and at some point may stop altogether. Under conventional forming and curing methods, the concrete takes a relatively long time to fully hydrate the cementitious materials. Since both the initial heat and moisture are quickly lost in conventional forms, it is typically recommended that concrete by moisture cured for 28 days to fully hydrate the concrete. However, moisture curing for 28 days is seldom possible to administer in commercial practice. Therefore, concrete poured in various applications in conventional forms seldom develops it maximum potential strength and durability.
Insulated concrete form systems are known in the prior art and typically are made from a plurality of modular form members. U.S. Pat. Nos. 5,497,592; 5,809,725; 6,668,503; 6,898,912 and 7,124,547 (the disclosures of which are all incorporated herein by reference in their entirety) are exemplary of prior art modular insulated concrete form systems. Full-height insulated concrete forms are also known in the prior art. U.S. Patent Application Publication No. 2011/0239566 and 2013/007432 (the disclosures of which are both incorporated herein by reference in their entirety) disclose full-height insulated concrete forms.
In accordance with the present invention, insulated concrete forms retain the heat of hydration and improve or accelerate concrete curing properties. However, prior art insulated concrete forms are designed to remain in place on the concrete structure. Conventional removable concrete forms are not insulated and therefore cannot retain the heat of hydration.
Prior art concrete forms have not been proposed or used as a method to cure concrete or to improve the performance and properties of concrete. The present invention has discovered that when the initial heat generated by the hydration of cementitious material is retained in an insulated concrete form, the concrete achieves a greater internal temperature and such temperature is sustained for much longer periods of time before it is lost to the environment. During this time, there is sufficient moisture in the concrete to more fully hydrate the cementitious material.
In the construction of certain concrete structures, such as high-rise buildings, it is desirable to use a concrete slip form. Concrete slip forming is a construction method in which concrete is poured into a continuously moving form. Slip forming is typically used for relatively tall structures, such as bridges, towers, high-rise buildings, silos, grain elevators and the like, as well as horizontal structures, such as aircraft runways and automotive highways. Slip forming enables continuous, uninterrupted, cast-in-place concrete structures, usually without joints, which thereby provides the concrete with superior performance characteristics relative to construction using discrete form elements. Slip forming relies on the relatively quick-setting properties of concrete, and requires a careful balance between quick-setting capability and concrete workability. Concrete needs to be sufficiently workable to be placed into a form and, if necessary, consolidated, such as by vibration. On the other hand, in slip forming concrete must be sufficiently quick-setting to emerge from the moving form with sufficient strength to support its own weight. Such strength is necessary because the freshly set concrete must not only permit the form to “slip,” but in vertical applications, the slip formed concrete must also support the weight of freshly poured concrete above it.
Due to the quick-setting properties required for slip forming concrete, concrete mixes employing reduced amounts of portland cement and relatively large amounts of supplementary cementitious or pozzolanic materials are not used for slip forming processes.